1. Field of the Invention
The present invention relates to a hydrogen storage material which can sorb and desorb hydrogen reversibly.
2. Description of the Related Art
In view of the environmental problems, such as the global warming due to the carbon dioxide emission, and the energy problems, such as the depletion of fossil fuel, a hydrogen energy has been attracting engineer's attention as a clean alternative energy recently. In order to put a hydrogen energy into practical applications, it is important to develop technologies for storing and transporting hydrogen safely. Carbon materials, such as activated carbons, fullerenes and carbon nanotubes, and hydrogen occluding alloys have been developed as hydrogen storage materials which can store hydrogen. Among them, hydrogen occluding alloys have been expected to be new transportable storage media because they can store a large amount of hydrogen as metallic hydrides in the form of solid, a safe form.
Many alloys have been known as hydrogen occluding alloys. For example, LaNi5 comprising a hexagonal system CaCu5 type crystal structure, and TiFe comprising a cubic system CsCl type crystal structure are practical hydrogen occluding alloys which can store and release hydrogen at room temperature, as disclosed in pp., 14-16, “SHINBAN SUISO KYUZO GOKIN—SONO BUSSEI TO OHYO (Hydrogen Occluding Alloy—The Physical Properties and Applications, New Edition)” written by OHSUMI Yasuaki and published by KABUSHIKI KAISHA AGNE GIJUTSU CENTER in Feb. 5, 1999. Moreover, Ca(Si2-xBx)y comprising a hexagonal system C12 type crystal structure wherein 0<x≦0.5 and 0.8≦y≦1.2, and Ca5Si3 comprising Cr5B3 type crystal structure are proposed as hydrogen occluding alloys whose constituent elements are Ca and Si existing abundantly as resources in nature, as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 10-8180 and in pp., 149-162, “J. Solid State Chem.,” 2001, Volume 159, respectively.
LaNi5 and TiFe include rare metals such as La, Ni and Ti, and accordingly suffer from drawbacks that it is difficult to secure the constituent elements as resources, and that they are very expensive. Moreover, LaNi5 and TiFe exhibit hydrogen absorptions of 2% or less per unit weight only, because they themselves are heavy hydrogen occluding alloys. In particular, it is difficult to hydrogenate TiFe initially. Consequently, in order to let TiFe absorb and desorb hydrogen, it is necessary to activate it at a high temperature under a high pressure in advance. On the other hand, it is difficult for Ca(Si2-xBx)y and Ca5Si3 to absorb hydrogen at around room temperature. In addition, in order to let Ca(Si2-xBx)y and Ca5Si3 absorb and desorb hydrogen, it is likewise necessary to activate them at a high temperature under a high pressure similarly to TiFe.